Method of butt-welding a coated steel plate

ABSTRACT

A method of butt-welding steel plates is provided. The method includes the steps of coating a first steel plate by dipping the first steel plate in a molten bath to obtain a first precoat upon the first steel plate. The first precoat includes a first intermetallic alloy layer and a first metal alloy layer, the first intermetallic alloy layer is topped by the first metal alloy layer. On a first face of the first steel plate, the first metal alloy layer is removed in a first area at a first periphery of the first steel plate, while at least part of the first intermetallic alloy layer in the first area remains. A second steel plate is coated by dipping the second steel plate in the molten bath or a further molten bath to obtain a second precoat upon the second steel plate. The second precoat includes a second intermetallic alloy layer and a second metal alloy layer, the second intermetallic alloy layer is topped by the second metal alloy layer. On a second face of the second steel plate, the second metal alloy layer is removed in a second area at a second periphery of the second metal plate. While at least part of the second intermetallic alloy layer in the second area remains. After removal of the first and second metal alloy layers, the first periphery of the first steel plate is butt-welded to the second periphery of the second steel plate to form a welded blank.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 14/070,132,filed Nov. 1, 2013 and U.S. patent application Ser. No. 13/311,731 filedon Dec. 6, 2011 which are continuations of U.S. Pat. No. 8,614,008issued on Dec. 24, 2013, which is a national stage application ofPCT/FR2007/00536 filed on Mar. 29, 2007 which claims the benefit ofPCT/FR2006/000898 filed on Apr. 19, 2006, the entire disclosures ofwhich are hereby incorporated by reference herein.

The invention concerns the fabrication of plates or blanks of coatedsteel intended to be welded and then heat treated to obtain parts havinggood mechanical characteristics and good corrosion resistance.

BACKGROUND

Some applications require steel parts combining high mechanicalstrength, high impact resistance and good corrosion resistance. Thistype of combination is particularly desirable in the automotive industrywhich requires a significant reduction in vehicle weight and excellentcapacity to absorb energy in the event of a collision. This can beachieved in particular by using steel with very good mechanicalcharacteristics having a martensitic or bainitic-martensiticmicrostructure: anti-intrusion, structural or safety components ofautomotive vehicles such as bumpers, door reinforcements, B-pillarreinforcements or roof reinforcements, for example, require the abovequalities.

Patent EP 0971044 discloses a fabrication method in which hot- orcold-rolled steel plate coated with aluminum of aluminum alloy is thestarting material. After shaping to produce a part, and before heattreatment at a temperature above A_(c1), the coating is heated to form asurface alloy by interdiffusion between the steel and the aluminumcoating. This alloy prevents decarburization of the metal and oxidationduring heat treatment in a furnace. It therefore eliminates thenecessity for furnaces containing a special atmosphere. The presence ofthis alloy also obviates certain surface operations on the treatedparts, such as shot blasting, which operations are necessary for plateshaving no coating. The parts are then cooled under conditions adapted toconfer a tensile strength that can exceed 1500 MPa.

With the aim of reducing vehicle weights, parts have been developedconsisting of steel blanks of different compositions or differentthicknesses continuously butt-welded together. These welded parts areknown as “butt-welded blanks” Laser beam welding is a preferred methodof assembling such blanks, exploiting the flexibility, quality andproductivity characteristics of the process. After these welded blankshave been cold-pressed, parts are obtained having mechanical strength,pressability, impact absorption properties that vary within the partsthemselves. It is therefore possible to provide the required propertiesat the appropriate location without imposing an unnecessary or costlypenalty on all of the parts.

The fabrication method described in patent EP 0971044 can be applied tobutt-welded blanks in the following manner: starting from steel plate,possibly of different compositions or thicknesses, and having a metalpre-coating, butt-welded blanks are obtained by a welding process. Thesewelded blanks then undergo heat treatment to form a surface alloy andare then hot-pressed and quenched. This produces quenched parts withthicknesses and intrinsic mechanical characteristics that vary andrepresent an ideal response to local loading requirements.

SUMMARY OF THE INVENTION

However, this fabrication method runs into considerable difficulties:when welding coated steel blanks, a portion of the initial surfacepre-coating is transferred into the molten area created by the weldingoperation. These exogenous metal elements are concentrated in particularby strong convection currents in the liquid metal. These elements aresegregated in particular in the interdendritic spaces in which theliquid fraction having the greatest concentration of dissolved elementsis located. If austenizing follows with a view to quenching the weldedblanks, these enriched areas become alloyed through interdiffusion withthe iron or other elements of the matrix and form intermetallic areas.On subsequent mechanical loading, these intermetallic areas tend to bethe site of onset of rupture under static or dynamic conditions. Theoverall deformability of the welded joints after heat treatment istherefore significantly reduced by the presence of these intermetallicareas resulting from welding and subsequent alloying and austenizing.

It is therefore desirable to eliminate the source of these intermetallicareas, namely the initial surface metal coating liable to be meltedduring butt-welding. However, eliminating this source itself gives riseto a serious problem: the precoated area on either side of the futurewelded joint can be eliminated, for example by a mechanical process. Thewidth of this area from which the pre-coating is removed must be atleast equal to that of the future area melted by welding so as not toencourage subsequent formation of intermetallic areas. In practice, itmust be much more than this, to allow for fluctuations in the width ofthe molten area during the assembly operation. Thus there exist afterthe welding operation areas on either side of the welded joint that nolonger have any surface metal pre-coating. During further alloying andaustenizing heat treatment, scale formation and decarburizing occurwithin these areas located next to the weld. These are areas that tendto corrode when the parts go into service because they are not protectedby any coating.

There is therefore a need for a fabrication process that prevents theformation of intermetallic areas within welded assemblies, which aresources of the onset of rupture.

There is also a need for a fabrication process such that the welded andheat treated parts have good corrosion resistance.

There is also a need for an economic fabrication process that can beintegrated without difficulty into existing welding lines and that iscompatible with subsequent pressing or heat treatment phases.

There is also a need for a product on which operations of butt-welding,then of heat treatment, pressing and quenching, lead to the fabricationof a part having satisfactory ductility and good corrosion resistance.One particular requirement is for a total elongation across the weldedjoint greater than or equal to 4%.

An object of the present invention is to solve the needs referred toabove.

The present invention therefore provides a plate consisting of a steelsubstrate and a precoat consisting of a layer of intermetallic alloy incontact with the substrate, topped by a layer of metal alloy. On atleast one precoated face of the plate, an area situated at the peripheryof the plate has the metal alloy layer removed.

The precoat is preferably an alloy of aluminum or based on aluminum.

The metal alloy layer of the precoat preferably comprises, by weight,from 8 to 11% of silicon, from 2 to 4% of iron, the remainder of thecompound being aluminum and inevitable impurities.

The width of the area from which the metal alloy layer has been removedis preferably between 0.2 and 2.2 mm.

The width of the area from which the metal layer has been removedpreferably varies.

The thickness of the intermetallic alloy layer is preferably between 3and 10 micrometers.

The area from which the metal alloy has been removed is preferablyproduced by partly eliminating the metal alloy layer on at least oneprecoated face of the plate by brushing.

The area from which the metal alloy has been removed can be produced bypartially eliminating the alloy layer on at least one precoated face ofthe plate by means of a laser beam.

The present invention also provides a welded blank obtained bybutt-welding at least two plates according to a preferred embodiment ofthe present invention, the welded joint being produced on the edgecontiguous with the area from which the metal alloy has been removed.

The present invention further provides a part obtained by heat treatmentand deformation of a welded blank according to a preferred embodiment ofthe present invention, the precoat being converted throughout itsthickness by the heat treatment into an intermetallic alloy compoundproviding protection against corrosion and decarburization of the steelsubstrate.

The present invention even further provides a plate, blank or partaccording to a preferred embodiment, the composition of the steelcomprising, by weight: 0.10%≦C≦0.5%, 0.5%≦Mn≦3%, 0.1%≦Si≦1%,0.01%≦Cr≦1%, Ti≦0.2%, Al≦0.1%, S≦0.05%, P≦0.1%, 0.0005%≦B≦0.010%, theremainder consisting of iron and inevitable impurities resulting fromthe production process.

The composition of the steel preferably comprises, by weight:0.15%≦C≦0.25%, 0.8%≦Mn≦1.8%, 0.1%≦Si≦0.35%, 0.01%≦Cr≦0.5%, Ti≦0.1%,Al≦0.1%, S≦0.05%, P≦0.1%, 0.002%≦B≦0.005%, the remainder consisting ofiron and inevitable impurities produced by the production process.

The present invention additionally provides a part according to apreferred embodiment wherein the microstructure of the steel ismartensitic, bainitic or bainitic-martensitic.

The present invention also provides a method that includes the steps ofcoating a steel plate to obtain a precoat including an intermetallicalloy layer topped by a metal alloy layer and, then, on at least oneface of the plate, removing the metal alloy layer in an area at theperiphery of the plate.

The width of the area may be preferably between 0.2 and 2.2 mm.

The invention further provides a method of fabricating a precoated steelplate that includes of coating a steel plate to obtain a precoat havingan intermetallic alloy layer topped by a metal alloy layer, on at leastone face of the plate, removing the metal alloy layer in an area nottotally contiguous with the periphery of the plate and cutting the platein a plane so that the area from which the metal alloy has been removedis at the periphery of the cut plate.

The width of the area from which the metal alloy has been removed andwhich is not totally contiguous with the periphery of the plate may bepreferably between 0.4 and 30 mm.

The precoating is preferably effected by dip coating with aluminum.

The layer is preferably removed by brushing.

In a preferred embodiment the layer is removed by the impact of a laserbeam on the precoat.

The invention also provides a method according to any one of the aboveembodiments in which the emissivity or reflectivity of the area overwhich the metal alloy layer is removed is measured, the measured valueis compared with a reference value characteristic of the emissivity orreflectivity of the metal alloy layer, and the removal operation isstopped when the difference between the measured value and the referencevalue is above a critical value.

The present invention also provides a method wherein the layer isremoved by means of a laser beam, characterized in that the intensity orwavelength of the radiation emitted at the point of impact of the laserbeam is measured, the measured value is compared with a reference valuecharacteristic of the emissivity of the metal alloy layer, and theremoval operation is stopped when the difference between the measuredvalue and the reference value is above a critical value.

The invention also provides a method wherein at least two platesfabricated according to any one of the above embodiments arebutt-welded, the welded joint being produced on the edge contiguous withthe area from which the metal alloy layer has been removed.

The width before welding of the area from which the metal layer has beenremoved at the periphery of the plate is preferably 20 to 40% greaterthan half the width of the weld.

The width of the area from which the metal alloy has been removed andwhich is not totally contiguous with the periphery of the plate ispreferably 20 to 40% greater than the width of a weld.

The present invention also provides a part fabrication method wherein awelded blank fabricated according to a preferred embodiment of thepresent invention is heated to form, by alloying between the steelsubstrate and the coating, an intermetallic alloy compound, and so as toconfer a partially or totally austenitic structure on the steel, thenthe blank is hot deformed to obtain a part. The part is cooled at a rateadapted to confer the target mechanical characteristics.

The rate of cooling is preferably above the critical rate formartensitic quenching.

In a preferred embodiment the welding is effected by a laser beam.

The welding is even more preferably effected by an electrical arc.

The present invention also provides a use of a plate, blank or partaccording to any one of the above embodiments for the fabrication ofstructural or safety parts for motorized terrestrial automotivevehicles.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will become apparent inthe course of the description given hereinafter by way of example andwith reference to the following appended figures:

FIG. 1 is a diagram showing one embodiment of plate according to thepresent invention before welding;

FIG. 2 is a diagram of a second embodiment of plate according to thepresent invention;

FIG. 3 is a diagram of an example of a butt-welded joint of the presentinvention;

FIG. 4 is a macrograph of a welded joint of the present invention afteraustenizing and alloying heat treatment;

FIG. 5 is a macrograph of a reference welded joint showing theappearance of harmful intermetallic areas within the molten metal; and

FIG. 6 is a macrograph of plate according to the present inventionbefore welding, from which plate the metal alloy has been removedlocally using a laser beam.

DETAILED DESCRIPTION

As explained above, total elimination of the metal coating on eitherside of the joint before welding has led to localized corrosionproblems. The inventors have surprisingly shown that eliminating aprecise portion of the coating solves the problems referred to above.

To explain the present invention, there are explained first certaincharacteristics of coated strip or plate usually produced by immersionin baths of molten zinc or aluminum or zinc or aluminum alloys.

These continuous, so-called “dip” methods yield the following generalmorphology of the coatings:

-   -   At the surface of the steel substrate of the plate an        intermetallic alloy a few micrometers thick is precipitated,        formed by a very fast reaction on immersion in the molten bath.        These intermetallic alloys being relatively fragile, inhibitors        are added to the molten bath in an attempt to limit the growth        of this layer. In the case of zinc or aluminum alloy coatings,        the alloys constituting this layer are often of the Fe_(x)Al_(y)        type, in particular Fe₂Al₅. In the case of zinc alloy coatings,        the presence of this aluminum-rich intermetallic layer is        explained by the fact that the zinc baths often contain a small        quantity of aluminum that plays an inhibitor role.

This layer of intermetallic alloys can sometimes be of a complex nature,for example divided into two intermetallic sub-layers, the sub-layer incontact with the substrate being richer in iron.

This layer of intermetallic alloys is topped by a metal alloy layer thecomposition of which is very close to that of the bath. A thicker orthinner metal layer is entrained by the plate as it leaves the moltenbath, and this thickness can be controlled by means of jets of air ornitrogen.

The inventors have shown that it is necessary to eliminate this layerlocally to solve the problems referred to above, which is particularlyadvantageous.

Consider more particularly FIG. 1, showing a plate of the presentinvention. The term plate is to be understood in a broad sense anddenotes in particular any strip or object obtained by cutting a strip, acoil or a sheet. In this particular example the plate has two faces andfour edges. The present invention is not limited to this rectangulargeometry, of course. FIG. 1 shows:

A steel substrate 1. This substrate can be of plate that is hot-rolledor cold-rolled, as a function of the required thickness, or of any otherappropriate form.

Superposed on the substrate, and in contact therewith, a pre-coating 2is present on the two faces of the part. This pre-coating itselfconsists of:

-   -   a layer of intermetallic alloy 3 in contact with the substrate        1. As already explained, this layer is formed by reaction        between the substrate and the molten metal of the bath.

The precoat is advantageously an aluminum alloy or aluminum-based. Thistype of precoat is particularly suitable for subsequent heat treatmentthat forms an intermetallic compound by interdiffusion with thesubstrate 1 and (see below) localized removal of the surface layer. Inparticular, the metal alloy of the precoat can contain 8 to 11% byweight of silicon and 2 to 4% of iron, the remainder consisting ofaluminum and inevitable impurities. Adding silicon enables reduction ofthe thickness of the intermetallic layer 3.

The periphery 5 of the plate is also shown. According to the invention,a portion 6 of the periphery does not carry the metal alloy layer 4 butretains the intermetallic alloy layer 3. This portion 6 is intended tobe placed in contact with another plate and then to be butt-welded in aplane defined by the edge 11 to form a blank.

In a first embodiment, the layer 4 is advantageously removed by means ofa brushing operation effected at the periphery 5: the material removedby the brush is essentially the surface layer, which has the lowesthardness, i.e. the metal alloy layer 4. The harder layer 3 will remainin place as the brush passes over it. Using an aluminum oraluminum-based precoat is particularly advantageous as the difference inhardness between the intermetallic alloy layer 3 and the metal layer 4is very large.

The person skilled in the art will know how to adapt the variousparameters specific to the brushing operation, such as the choice of thekind of brush, the speed of rotation and of relative movement intranslation, the pressure perpendicular to the surface, to carry out theremoval as completely and quickly as possible, adapting them to theparticular nature of the precoat. For example, a wire brush mounted on arotary shaft driven in translation parallel to the edge of the part 6could be used.

In a second embodiment, the layer 4 is removed by a laser beam directedtoward the periphery of the plate: interaction between this high energydensity beam and the precoat causes vaporization and expulsion of thesurface of the precoat. Given the different thermal and physicalproperties of the metal alloy layer 4 and the intermetallic layer 3, theinventors have shown that a succession of short laser pulses withappropriate parameters leads to selective ablation of the metal layer 4,leaving the layer 3 in place. The interaction of a pulsed laser beamdirected toward the periphery of a coated plate and moved in translationrelative to that plate therefore removes the peripheral metal layer 4.The person skilled in the art will know how to adapt the variousparameters, such as the choice of laser beam, the incident energy, thepulse duration, the speed of relative movement in translation betweenthe beam and the plate, and the focusing of the beam onto the surface tocarry out the ablation as quickly and completely as possible, adaptingthem to the particular nature of the precoat. For example, a Q-switchlaser could be used, having a nominal power of a few hundred watts anddelivering pulses with a duration of the order of 50 nanoseconds. Thewidth of the removal area 6 can naturally be varied by means ofsuccessive contiguous ablations.

The width of the area 6 from which the metal layer has been removed mustbe adjusted to enable:

-   welding with no introduction of any element of the precoat into the    molten area,-   sufficient corrosion resistance of the welded assembly after    subsequent alloying and austenizing heat treatment.

The inventors have shown that the above conditions are satisfied if thewidth of the area 6 is 20% to 40% greater than half the width of themolten area created when butt-welding blanks

The minimum value of 20% ensures that the precoat is not introduced intothe molten metal during welding, and the value of 40% ensuressatisfactory corrosion resistance.

Given the welding conditions for plate from 1 to 3 mm thick, the widthof the area 6 is between 0.2 and 2.2 mm.

This situation is represented in FIG. 3, which shows diagrammatically insection after welding plate comprising a precoat 2 formed of anintermetallic alloy layer 3 and a metal layer 4. The molten area 10 hasits axial plane 9 in the welding direction. The dashed lines show theinitial extent of an area 6 melted by the welding operation.

FIG. 3 illustrates the situation in which the weld is globallysymmetrically on the two opposite faces of the plate. Under theseconditions, the width of the area 6 is exactly the same on both faces.However, as a function of the welding process used and the parameters ofthat process, the weld can have an asymmetrical appearance. According tothe invention, the width of the area 6 can then be coordinated to thisasymmetry so that this width is slightly greater than half the width ofthe molten area 10 on each of the respective two faces. Under theseconditions, the width of the area 6 differs from that of the area 6′shown in FIG. 3.

If welding conditions evolve during an assembly operation, for exampleto take account of local modification of geometry or thickness, thewidth of the area 6 can also be coordinated with the correspondingvariation of the width of the molten area along the welded periphery ofthe plate. The width of the area 6 naturally increases if localconditions lead to the formation of a wider weld.

In the case of welding two coated plates of different thickness, thewidth of the area 6 can also be different on the welded peripheralportion of each of the two plates.

In a variant of the invention shown in FIG. 2, the layer 4 is removedover an area 7 of a coated plate that is not totally contiguous with theperiphery 5 of the plate. The plate is then cut in an axial plane 8perpendicular thereto, for example by a slitting process. A plate asshown in FIG. 1 is then obtained. The width removed is 20% to 40%greater than the width of the molten area that would be produced by awelding operation in the axial plane 8.

In one variant of the invention, the width removed is between 0.4 and 30mm. The minimum value corresponds to a width such that cutting in theaxial plane 8 produces two plates having a very narrow removal area 0.2mm wide on each of the two plates. The maximum value of 30 mmcorresponds to a removal width well suited to industrial tools forperforming such removal. A subsequent cutting operation can be effected,not on the axial plane 8 situated in the middle of the removal area, butat a location adapted to produce a plate whose removal width is slightlygreater than half the width of the molten area produced by a weldingoperation, defined by the conditions of the invention.

As explained above, the removed widths ensure that the metal coating isnot introduced into the molten metal during subsequent welding of theplate and also that the welded blank is corrosion resistant after heattreatment.

Removal of the metal layer 4 can be monitored by means of micrographicexamination. However, it has also been shown that the efficiency of theremoval operation can be checked very quickly by optical inspection:there is a difference in appearance between the metal layer 4 and theunderlying intermetallic layer 3, which is darker. The removal operationmust therefore continue and be stopped when there is seen in the area 6a significant change of tone relative to the surface coating. It istherefore possible to monitor removal by spectrometer reflectivity oremissivity measurement: the area 6 is illuminated by a light source, oneor more optical sensors being directed towards this area. The measuredvalue corresponds to the reflected energy. That value is compared with areference value corresponding to the emissivity or reflectivity of themetal layer 4 or with a value measured by another sensor directed towardthe metal layer. It is also possible to measure the variation of thereflected energy as a function of time. If the layer 6 is flush with thesurface, the energy collected is lower than that corresponding to themetal alloy layer 4. The precise moment at which the removal operationreaches the layer 3 can therefore be determined by previous calibration.

In the case of coating removal by laser ablation, it is also possible toanalyze the intensity or the wavelength of the radiation emitted at thepoint of impact of the laser beam on the precoated plate. The intensityand the wavelength are modified when the layer 4 has been eliminated andthe laser beam impacts on the layer 3. The thickness of the layerremoved can therefore be monitored in the following manner: theintensity or the wavelength of the radiation emitted at the point ofimpact of the laser beam is measured, that measured value is comparedwith a reference value characteristic of the emissivity of the metalalloy layer 4, and the removal operation is stopped when the differencebetween the measured value and the reference value is above apredetermined critical value.

Depending on specific constraints, this step of removing the metal alloylayer can be carried out at various stages of the production process,and in particular:

-   either after unwinding coils fabricated on continuous rolling mill    trains, before cutting to form a smaller format plate,-   or before welding the cut plate.

In the method of the invention, a hot- or cold-rolled steel plate withthe following composition by weight is the starting material: carboncontent between 0.10 and 0.5%, and preferably between 0.15 and 0.25% byweight. This element impacts greatly on the quenchability and on themechanical strength obtained after cooling that follows the alloying andaustenizing of the welded blanks Below a content of 0.10% by weight, thequenchability is too low and the strength properties are insufficient.In contrast, beyond a content of 0.5% by weight, the risk of defectsappearing during quenching is increased, especially for the thickestparts. A carbon content between 0.15 and 0.25% produces a tensilestrength between about 1250 and 1650 MPa.

Apart from its role as a deoxidant, manganese also has a significanteffect on quenchability, in particular if its concentration by weight isat least 0.5% and preferably 0.8%. However, too great a quantity (3% byweight, or preferably 1.8%) leads to risks of excessive segregation.

The silicon content of the steel must be between 0.1 and 1% by weight,and preferably between 0.1 and 0.35%. Apart from its role of deoxidizingthe liquid steel, this element contributes to hardening. Its contentmust nevertheless be limited to avoid excessive formation of oxides andto encourage coatability.

Beyond a content above 0.01%, chromium increases quenchability andcontributes to obtaining high strength after the hot forming operation,in the various portions of the part after cooling following theaustenizing and alloying heat treatment. Above a content equal to 1%(preferably 0.5%), the contribution of chromium to obtaining homogeneousmechanical properties reaches saturation.

Aluminum favors deoxidation and precipitation of nitrogen. In amountsabove 0.1% by weight, coarse aluminates form during production, which isan incentive to limit the content to this value.

Excessive quantities of sulfur and phosphorus lead to increasedweakness. For this reason it is preferable to limit their respectivecontents to 0.05 and 0.1% by weight.

Boron, the content of which must be between 0.0005 and 0.010% by weight,and preferably between 0.002 and 0.005% by weight, has a large impact onquenchability. Below a content of 0.0005%, insufficient effect isachieved vis à vis quenchability. The full effect is obtained for acontent of 0.002%. The maximum boron content must be less than 0.010%,and preferably 0.005%, in order not to degrade toughness.

Titanium has a high affinity for nitrogen and therefore contributes toprotecting the boron so that this element is found in free form to haveits full effect on quenchability. Above 0.2%, and more particularly0.1%, there is however a risk of forming coarse titanium nitrides in theliquid steel, which have a harmful effect on toughness.

After preparation of the plate according to any of the methods describedabove, they are assembled by welding to obtain a welded blank. More thantwo plates can naturally be assembled to fabricate complex finishedparts. The plates can be of different thickness or composition toprovide the required properties locally.

Welding is effected after placing the plates edge-to-edge, the areaswith no metal alloy layer being in contact with each other. Welding istherefore effected along the edge contiguous with the areas 6 where themetal alloy layer has been removed.

In the context of the invention, any continuous welding means can beused appropriate to the thicknesses and to the productivity and qualityconditions required for the welded joints, and in particular:

-   laser beam welding,-   electric arc welding, and in particular the GTAW (Gas Tungsten Arc    Welding), plasma, MIG (Metal Inert Gas) or MAG (Metal Active Gas)    processes.

Under the conditions of the invention, the welding operation does notlead to remelting of a portion of the metal coating 4, elements whereofwould thereafter be found in the molten area. Only a minimal quantity ofthe intermetallic alloy layer 3 is remelted by this operation into themolten area. As the following example shows, this very limited quantityhas no influence on the metallurgical quality or the mechanicalproperties of the welded joint after alloying and austenizing heattreatment.

The welded blank is then heated to bring about conjointly:

-   A surface alloying treatment in which elements of the steel    substrate, in particular iron, manganese and silicon, diffuse into    the precoat. This forms a surface intermetallic alloy compound the    melting point of which is significantly higher than that of the    metal alloy layer 4. The presence of this compound during heat    treatment prevents oxidation and decarburization of the underlying    steel.-   Austenizing of the base steel, either partial or total. The heating    is advantageously effected in a furnace so that the part reaches a    temperature between Ac1 and Ac3+100° C. Ac1 and Ac3 are respectively    the start and end temperatures of the austenitic transformation that    occurs on heating. According to the invention, this temperature is    maintained for a time greater than or equal to 20 s so as to render    uniform the temperature and microstructure at the various points of    the part.

Under the conditions of the present invention, during this heatingphase, no brittle intermetallic areas are formed within the moltenmetal, which would be harmful to the mechanical properties of the part.

This is followed by hot deformation of the blank to its final shape as apart, this step being favored by the reduction of the creep limit andthe increase of the ductility of the steel with temperature. Startingfrom a structure that is partly or totally austenitic at hightemperature, the part is then cooled under appropriate conditions toconfer the target mechanical characteristics: in particular, the partcan be held in a tooling during cooling, and the tooling can itself becooled to encourage the evacuation of heat. To obtain good mechanicalproperties, it is preferable to produce martensitic, bainitic orbainitic-martensitic microstructures.

In the area 6 on either side of the welded joint, the intermetalliclayer 3, which is between 3 and 10 micrometers thick before heattreatment, is alloyed with the steel substrate and produces goodcorrosion resistance.

EXAMPLE

The following embodiments show by way of example other advantagesconferred by the present invention. They concern a cold-rolled steelstrip 1.5 mm thick, with the following composition by weight:

TABLE 1 Composition of the steel (% by weight) C Mn Si S P Al Cr Ti B0.224 1.160 0.226 0.005 0.013 0.044 0.189 0.041 0.0031

The steel strip was precoated by dipping it in a molten bath of analuminum alloy containing 9.3% of silicon and 2.8% of iron, theremainder consisting of aluminum and inevitable impurities. The stripwas then cut into plates with a format of 300×500 mm². These have oneach face a precoat comprising a layer of intermetallic alloy comprisingmostly Fe₂Al₃, Fe₂Al₅ and Fe_(x)Al_(y)Si_(z). This 5 micrometers thicklayer in contact with the steel substrate has a 20 micrometers thicklayer of Al—Si metal alloy on top of it.

Before laser beam welding, four different preparation methods were used:

-   Method I (according to the present invention): the Al—Si metal alloy    layer was removed by longitudinal brushing over a width of 1.1 mm    from the edge of the plate, on the 500 mm long side. Brushing was    effected in exactly the same way on both faces using an 80 mm    diameter “Spiraband” wire brush mounted on an angled rotary system,    guided in movement in translation on a counterweight bench. The    brushing force is approximately 35 N at the point of brush/blank    contact, and the speed of movement of the brush 10 m/min. This    brushing eliminates the metal alloy layer, leaving only the 5    micrometer intermetallic alloy layer in the brushed area.-   Method II (according to the present invention): the Al—Si metal    alloy layer was removed by laser ablation over a width of 0.9 mm    from the edge of the plate. The laser ablation was carried out in    exactly the same way on both faces using a Q-switch laser with a    nominal energy of 450 W delivering 70 ns pulses. The pulse energy is    42 mJ. The constant speed of movement in translation of the laser    beam relative to the plate is 20 m/min. FIG. 6 shows that this laser    ablation eliminates the metal alloy layer 4 leaving only the 5    micrometer intermetallic alloy layer 3 in the treated area.-   Method R1 (not according to the invention): all of the precoat,    comprising the metal alloy layer and the intermetallic alloy, was    mechanically removed over a width of 1.1 mm, and therefore identical    to that of method 1, by means of a carbide plate type tool for fast    machining, in longitudinal translation. As a result, subsequent    welding is carried out in an area with all of the precoat removed on    either side of the joint.-   Method R2 (not according to the invention): laser welding was    effected on precoated plate with no particular preparation of the    periphery.

The above plates were laser beam welded under the following conditions:nominal power: 6 kW, welding speed: 4 m/minute. Given the width of theweld, in method I, there is found the presence of an area with no metalalloy over a width of approximately 0.3 mm following production of thewelded joints.

The welded blanks were subjected to alloying and austenizing heattreatment including heating to a temperature of 920° C., which wasmaintained for 7 minutes. These conditions lead to complete austenitictransformation of the steel of the substrate. During this heating andconstant temperature phase, it is found that the aluminum-silicon-basedprecoat forms an intermetallic compound throughout its thickness byalloying with the base steel. This alloy coating has a high meltingpoint and a high hardness, features high corrosion resistance, andprevents oxidation and decarburization of the underlying base steelduring and after the heating phase.

After the phase of heating to 920° C., the parts were hot-deformed andcooled.

Subsequent cooling between jigs yielded a martensitic structure. Thetensile R_(m) of the steel substrate obtained after such treatment isabove 1450 MPa.

The following techniques were then used to characterize the weldedjoints in the parts obtained in this way:

-   Micrographic sections show the presence of any intermetallic areas    within the welded joints.-   Mechanical tension tests across welded joints in samples 12.5×50 mm²    determines the tensile strength R_(m) and the total elongation.

Accelerated corrosion tests were carried out according to the DIN 50021,50017, and 50014 standards. These tests include, following salt mistspraying, cycles alternating dry phases at 23° C. and wet phases at 40°C.

Table 2 sets out the results of these characterizations:

TABLE 2 Welded joint characteristics after heat treatment Fragileintermetallic areas within Corrosion Method welded joints Rm (MPa) A(%)resistance I (according to the None >1450 ≧4 ◯ present invention) II(according to the None >1450 ≧4 ◯ present invention) R1 (not accordingto the None >1450 ≧4 ● invention) R2 (not according to the Present 1230≦1 ◯ invention) ◯: Satisfactory ●: not satisfactory

Under the quenching conditions required after heat treatment, themicrostructure of the base metal and the molten area during welding istotally martensitic with the above four methods.

In the case of method I of the invention, the melted area contains nointermetallic area, as FIG. 4 shows.

On the other hand, in the method R2, note the presence of intermetallicareas (see FIG. 5), in particular towards the periphery of the meltedarea where the elements of the precoat were concentrated by spontaneousconvection currents in the liquid bath caused by a Marangoni effect.These large intermetallic areas, which can be oriented substantiallyperpendicularly to the mechanical load, act as stress concentration andonset of rupture effects. Elongation in the crosswise direction is inparticular reduced by the presence of these intermetallic areas: in theabsence of these areas, the elongation is above 4%. It drops to below 1%when they are present.

No significant difference in mechanical characteristics (strength andelongation) is noted between the method I of the invention and themethod R1. This indicates that the thin layer of intermetallic alloyleft in place by brushing and remelted by welding does not lead to theformation of brittle areas within the molten metal, as FIG. 4 shows.

In the case of the method R1, corrosion resistance is reduced: the steelis totally bared on either side of the welded joint by the total removalof the precoat. Lacking corrosion protection, red rust is then seen toappear in the heat-affected areas on either side of the weld.

Thus the method of the invention simultaneously achieves good ductilityof the welded joint after treatment and good corrosion resistance.

Depending on the composition of the steel, in particular its carboncontent and its manganese, chromium and boron content, the maximumstrength of the parts can be adapted to the target use. Such parts willbe used with profit for the fabrication of safety parts, and inparticular anti-intrusion or underbody parts, reinforcing bars,B-pillars, for the construction of automotive vehicles.

What is claimed is:
 1. A method of butt-welding steel plates, the methodcomprising: coating a first steel plate by dipping the first steel platein a molten bath to obtain a first precoat upon the first steel plate,wherein the first precoat consists of a first intermetallic alloy layerand a first metal alloy layer, wherein the first intermetallic alloylayer is topped by the first metal alloy layer; on a first face of thefirst steel plate, removing the first metal alloy layer in a first areaat a first periphery of the first steel plate, while leaving at leastpart of the first intermetallic alloy layer in the first area; coating asecond steel plate by dipping the second steel plate in the molten bathor a further molten bath to obtain a second precoat upon the secondsteel plate, wherein the second precoat consists of a secondintermetallic alloy layer and a second metal alloy layer, wherein thesecond intermetallic alloy layer is topped by the second metal alloylayer; on a second face of the second steel plate, removing the secondmetal alloy layer in a second area at a second periphery of the secondmetal plate, while leaving at least part of the second intermetallicalloy layer in the second area; after removal of the first and secondmetal alloy layers, butt-welding the first periphery of the first steelplate to the second periphery of the second steel plate to form a weldedblank.
 2. The method as recited in claim 1 wherein the first and secondintermetallic alloy layers which contact the weld are remelted andwherein the first and second metal alloy layers are not remelted duringwelding.
 3. The method as recited in claim 1 wherein the butt-welding isperformed via laser beam welding.
 4. The method as recited in claim 1wherein the butt-welding is performed via electric arc welding.
 5. Themethod as recited in claim 1 wherein the first metal alloy layer isremoved by laser ablation.
 6. The method as recited in claim 1 whereinthe first metal alloy layer is removed by brushing.
 7. The method asrecited in claim 1 wherein the coating of the first steel plate iseffected by dip coating with aluminum.
 8. The method as recited in claim1 wherein the coating of the first steel plate is effected by dipcoating with zinc.
 9. The method as recited in claim 1 wherein the firstmetal alloy layer comprises aluminum, and, by weight: from 8 to 11% ofsilicon; and from 2 to 4% of iron.
 10. The method as recited in claim 1wherein the butt-welding forms a weld symmetrical on the first andsecond faces and opposing faces.
 11. The method as recited in claim 1wherein the butt-welding forms a weld asymmetrical on the first andsecond faces and opposing faces.
 12. The method as recited in claim 1wherein the first steel plate has a different thickness than the secondsteel plate.
 13. The method as recited in claim 1 wherein the first andsecond metal alloy layers are removed sufficiently so that the first andsecond metal alloy layers are not introduced into a weld pool during thebutt-welding.
 14. The method as recited in claim 13 wherein a width ofthe first area is 20% to 40% greater than half a width of the weld pool.15. The method as recited in claim 13 wherein a width of the first areais between 0.2 and 2.2 mm.
 16. The method as recited in claim 1 whereinthe first metal alloy layer is removed while leaving all of the firstintermetallic alloy layer in the first area.